examples: Follow best practices and established naming conventions

This is a nitpick but from my experience and understanding the best
practice for label key naming is to use one word, otherwise using an
underscore. Since this is an example users tend to copy, I think
correcting it might be a good idea.

examples/customlabels/main.go

@@ -33,7 +33,7 @@ func main() {
 
 	// Create a new registry.
 	reg := prometheus.NewRegistry()
-	prometheus.WrapRegistererWith(prometheus.Labels{"serviceName": "my-service-name"}, reg).MustRegister(
+	prometheus.WrapRegistererWith(prometheus.Labels{"service": "my-service-name"}, reg).MustRegister(
 		collectors.NewGoCollector(),
 		collectors.NewProcessCollector(collectors.ProcessCollectorOpts{}),
 	)